1. Field of the Invention
The present invention relates to a dielectric resonator, and more particularly, to a dielectric resonator having a thin film multi-layered electrode and a dielectric filter and duplexer including the dielectric resonator which are applicable to communication apparatuses and the like for use in base stations of a cellular telephone system, and a communication device including the dielectric filter.
2. Description of the Related Art
FIG. 9 is an exploded perspective view of a dielectric filter which is related to the present invention. The dielectric filter is described in the co-pending U.S. patent application Ser. No. 08/924,040 filed Aug. 29, 1997, now U.S. Pat. No. 6,052,041, the disclosures of which are incorporated by reference. However, the art disclosed in the '040 application was not publicly known on the filing date of Japanese Patent Application No. H10-38810, the priority application on which the present invention is based. In FIG. 9, a dielectric filter 110 comprises a metallic case 111, dielectric resonators 112 disposed inside of the case 111, a ground plate 113, coupling probes 114, and external connectors 115 attached to the outside wall of the case and connected to the probes 114, respectively. The case 111 comprises a trunk 111a and an upper lid 111b. On the upper and under sides of each dielectric resonator 112, thin film multilayered electrodes are formed, respectively. Each thin film multilayered electrode is composed of dielectric layers and conductor layers alternately laminated to each other. The detailed structure of the thin film multilayered electrode is described in the co-pending U.S. patent application Ser. No. 08/604,952 filed Feb. 27, 1996, now pending (international application number PCT/JP94/00357). The disclosures of the '952 and '357 applications are incorporated herein by reference.
The ground plate 113 is made of a metallic plate. For the purpose of reducing the temperature dependency of the filter characteristics, the ground plate has a coefficient of linear expansion equal to that of the dielectric resonators 112. The dielectric resonators 112 are fixed to the ground plate 113 by soldering. The ground plate 113 is sandwiched between the trunk 111a and the lid 111b, and thereby, the dielectric resonators 112 are disposed in the case 111. The ground plate 113 is placed on the trunk 111a so that gaps are formed between the dielectric resonators 112 and the trunk 111a.
Each coupling probe 114 is made of a metallic wire and extends into the gap between a corresponding dielectric resonator 112 and the trunk 111a. The coupling probe 114 and the dielectric resonator 112 are capacitively coupled. The two dielectric resonators together function as a dielectric filter. If the external connectors 115 are connected through a .lambda./4 line 117, the dielectric filter functions as a band elimination dielectric filter.
In order to fix the dielectric resonators to the ground plate, soldering techniques are generally used as described above. For the purpose of making the best use of the characteristics of the thin film multilayered electrodes, it is preferable to consider the following points. FIG. 10 is a cross-sectional view taken along a line W--W of FIG. 9. Solder is coated onto the upper side of the dielectric resonator 112 with a soldering iron and retained there so as to short-circuit the under side of the ground plate and the resonator. As a result, the respective electrodes of the thin film multilayered electrode are short-circuited. The solder may be permeated between the resonator 12 and the ground plate 113 according to re-flow techniques. However, excess solder reaches the side of the resonator to short-circuit the respective electrodes of the thin film multilayered electrode.
The thin film multilayered electrode is provided for the purpose of enhancing the non-loaded Q of the dielectric filter, by reduction of the conductor loss in the electrode due to the skin effect. The thicknesses of the respective electrode layers are strictly set. Therefore, the short-circuiting of the respective electrode layers as described above should be avoided.
In the event that a stress, caused by external vibration or impact, is applied to the ground plate, the stress is transmitted to one or more side edges of the thin film multilayered electrode, since the ground plate is flat. The thin film multilayered electrode is susceptible to being peeled apart at its side edges. Thus, there is a possibility that the thin film multilayered electrode may be peeled apart, or peeled off of the ground plate, at a side edge thereof.